Geology topics

A.E. Egger

Publications and source records attributed to A.E. Egger.

5 recordsLinked to original sources

USGS2018

A guided inquiry approach to learning the geology of the U.S

A guided inquiry exercise has been developed to help teach the geology of the U.S. This exercise is intended for use early in the school term when undergraduate students have little background knowledge of geology. Before beginning, students should be introduced to rock types and have a basic understanding of geologic time. This exercise uses three maps: the U.S. Geological Survey's "A Tapestry of Time and Terrain" and "Landforms of the Conterminous United States" maps, and a geologic map of the United States. Using these maps, groups of 3 to 5 students are asked to identify between 8 and 12 geologic provinces based on topography, the age of rocks, and rock types. Each student is given a blank outline map of the contiguous U.S. and each group is given a set of the three maps and colored pencils; as a group, students work to define regions in the U.S. with similar geology. A goal of 8 to 12 geologic provinces is given to help establish the level of detail being asked of students. One member of each group is asked to present their group's findings to the class, describing their geologic provinces and the reasoning behind their choices.

Journal of Geoscience Education↗

USGS2013

Correlation of geothermal springs with sub-surface fault terminations revealed by high-resolution, UAV-acquired magnetic data

There is widespread agreement that geothermal springs in extensional geothermal systems are concentrated at fault tips and in fault interaction zones where porosity and permeability are dynamically maintained (Curewitz and Karson, 1997; Faulds et al., 2010). Making these spatial correlations typically involves geological and geophysical studies in order to map structures and their relationship to springs at the surface. Geophysical studies include gravity and magnetic surveys, which are useful for identifying buried, intra-basin structures, especially in areas where highly magnetic, dense mafic volcanic rocks are interbedded with, and faulted against less magnetic, less dense sedimentary rock. High-resolution magnetic data can also be collected from the air in order to provide continuous coverage. Unmanned aerial systems (UAS) are well-suited for conducting these surveys as they can provide uniform, low-altitude, high-resolution coverage of an area without endangering crew. In addition, they are more easily adaptable to changes in flight plans as data are collected, and improve efficiency. We have developed and tested a new system to collect magnetic data using small-platform UAS. We deployed this new system in Surprise Valley, CA, in September, 2012, on NASA's SIERRA UAS to perform a reconnaissance survey of the entire valley as well as detailed surveys in key transition zones. This survey has enabled us to trace magnetic anomalies seen in ground-based profiles along their length. Most prominent of these is an intra-basin magnetic high that we interpret as a buried, faulted mafic dike that runs a significant length of the valley. Though this feature lacks surface expression, it appears to control the location of geothermal springs. All of the major hot springs on the east side of the valley lie along the edge of the high, and more specifically, at structural transitions where the high undergoes steps, bends, or breaks. The close relationship between the springs and structure terminations revealed by this study is unprecedented. Collecting magnetic data via UAS represents a new capability in geothermal exploration of remote and dangerous areas that significantly enhances our ability to map the subsurface.

California↗

USGS2011

Oligocene and Miocene arc volcanism in northeastern California: evidence for post-Eocene segmentation of the subducting Farallon plate

The Warner Range in northeastern California exposes a section of Tertiary rocks over 3 km thick, offering a unique opportunity to study the long-term history of Cascade arc volcanism in an area otherwise covered by younger volcanic rocks. The oldest locally sourced volcanic rocks in the Warner Range are Oligocene (28–24 Ma) and include a sequence of basalt and basaltic andesite lava flows overlain by hornblende and pyroxene andesite pyroclastic flows and minor lava flows. Both sequences vary in thickness (0–2 km) along strike and are inferred to be the erosional remnants of one or more large, partly overlapping composite volcanoes. No volcanic rocks were erupted in the Warner Range between ca. 24 and 16 Ma, although minor distally sourced silicic tuffs were deposited during this time. Arc volcanism resumed ca. 16 Ma with eruption of basalt and basaltic andesite lavas sourced from eruptive centers 5–10 km south of the relict Oligocene centers. Post–16 Ma arc volcanism continued until ca. 8 Ma, forming numerous eroded but well-preserved shield volcanoes to the south of the Warner Range. Oligocene to Late Miocene volcanic rocks in and around the Warner Range are calc-alkaline basalts to andesites (48%–61% SiO 2 ) that display negative Ti, Nb, and Ta anomalies in trace element spider diagrams, consistent with an arc setting. Middle Miocene lavas in the Warner Range are distinctly different in age, composition, and eruptive style from the nearby Steens Basalt, with which they were previously correlated. Middle to Late Miocene shield volcanoes south of the Warner Range consist of homogeneous basaltic andesites (53%–57% SiO 2 ) that are compositionally similar to Oligocene rocks in the Warner Range. They are distinctly different from younger (Late Miocene to Pliocene) high-Al, low-K olivine tholeiites, which are more mafic (46%–49% SiO 2 ), did not build large edifices, and are thought to be related to backarc extension. The Warner Range is ∼100 km east of the axis of the modern arc in northeastern California, suggesting that the Cascade arc south of modern Mount Shasta migrated west during the Late Miocene and Pliocene, while the arc north of Mount Shasta remained in essentially the same position. We interpret these patterns as evidence for an Eocene to Miocene tear in the subducting slab, with a more steeply dipping plate segment to the north, and an initially more gently dipping segment to the south that gradually steepened from the Middle Miocene to the present.

California↗

USGS2010

Timing and nature of tertiary plutonism and extension in the Grouse Creek Mountains, Utah

The Grouse Creek -Albion-Raft River metamorphic core complex in northwestern Utah and southern Idaho is characterized by several Tertiary plutons with a range of ages and crosscutting relations that help constrain the timing of extensional deformation. In the Grouse Creek Mountains , at least three distinct, superimposed, extension -related Tertiary deformational events are bracketed by intrusive rocks, followed by a fourth event: motion on range-bounding faults. The Emigrant Pass plutonic complex was emplaced at depths of less than 10 km into Permianage rocks. SHRIMP U-Pb zircon analysis indicates a three-stage intrusion of the complex at 41.3 ± 0.3 Ma, 36.1 ± 0.2 Ma, and 34.3 ± 0.3 Ma. The two youngest phases represent distinctly younger intrusive event(s) than the oldest phase, separated by more than 5 m.y. The oldest phase cuts several metamorphosed and deformed younger-on-older faults, providing a pre-41 Ma age bracket for oldest extension -related deformation in the region. The youngest phase(s) are interpreted to have been intruded during development of a map-scale, N-S-trending recumbent fold, the Bovine Mountain fold, formed during vertical shortening of roof rocks during intrusion. This second event folded older normal faults that are likely pre-41 Ma. Zircons from the youngest part of the pluton show inheritance from Archean basement (~2.5 Ga) and from its Proterozoic sedimentary cover (~1.65 Ga). The Red Butte pluton, emplaced at 15-20 km depth, intruded highly metamorphosed Archean orthogneiss at 25.3 ± 0.5 Ma; cores of some zircons yield latest Archean ages of 2.55 Ga. The pluton is interpreted to have been intruded during a third deformational and metamorphic event that resulted in vertical flattening fabrics formed during NW to EW stretching, ultimately leading to thinning of cover and top-to-the west motion on the Ingham Pass fault. The Ingham Pass fault represents an important structure in the Grouse Creek Mountains , as it juxtaposes two parts of the crust that apparently resided as much as 10 km apart ( in depth) at times as young as the Miocene. The varied structural, metamorphic, and intrusive relations observed in the Grouse Creek Mountains reflect their formation at different levels within the crust. Data from these various levels argue that plutonism has been a key mechanism for transferring heat into the middle and upper crust, and localizing strain during regional extension . Interestingly, events documented here correlate in a broad way with cooling events documented in the Raft River Mountains , although plutons are not exposed there. Major and trace element geochemistry imply a crustal component in all of the studied plutons, indicating significant degrees of crustal melting at depth during extension , and point to mantle heat sources during the timespan of Basin and Range extension as the cause of melting. Basin and Range faulting and final uplift of the range is recorded by apatite fission track ages, averaging 13.4 Ma, and deposition of about 2 km of syn-faulting basin fill deposits along the Grouse Creek fault mapped along the western flank of the range. Similar apatite ages from the Albion Mountains to the north indicate that the western side of the Albion-Raft River- Grouse Creek core complex behaved as a single rigid crustal block at this time.

Utah↗

USGS2009

Provenance and palaeogeographic implications of Eocene-Oligocene sedimentary rocks in the northwestern Basin and Range

A thick sequence of uppermost Eocene to lower Oligocene volcaniclastic and sedimentary rocks is exposed at the base of the Warner Range in northeastern California. This isolated exposure provides insight into the palaeogeographic setting of the northwestern Basin and Range during this time period. Significant thinning of the unit over 35km of lateral exposure and predominantly volcanic clast compositions suggest that the sequence was deposited in an alluvial plain adjacent to a volcanic arc. Palaeocurrent indicators in the conglomerates define a NNE transport direction. Detrital zircon analysis on coarse sandstones and dating of individual granite cobbles show a range of ages consistent with a local, volcanic source area primarily from the SSW with some far-travelled input from northern Nevada; the far-travelled component increases in influence as the unit thins to the north. Comparison with other sedimentary sequences of Eocene age and integration with palaeofloral and geophysical data help to define drainage divides, and suggest that this sequence accumulated in a relatively isolated, intra-arc basin. This localized accumulation differs markedly from contemporaneous drainages to the south that transported material westwards from central Nevada to the palaeoshoreline, and suggests that ongoing volcanism had a strong influence on palaeogeography in this region during the Eocene and Oligocene.

International Geology Review↗